The present invention is directed to a method for the transmission of data in an ATM transmission system as well as to an ATM transmission system, particularly an ATM broadband transmission system.
Many new transmission or switching principles for various types of transmission in communication networks have been developed during the course of the rapid development of communications technology in recent years. The STM transmission principle (synchronous transfer mode) deals with a synchronous transfer or transmission method, in which the data of various data channels are serially transmitted within different time slots, and the individual time slots are combined into frames. A frame synchronization word is transmitted for the synchronization of each and every frame, so that each time slot of a frame allocated to a specific date channel exhibits a fixed time spacing from the frame synchronization word. Each time slot can contain a relatively small number of bits, for example 8 bits, and appears at constant time intervals. However, highly different bit rates cannot be uniformly governed with the assistance of this STM principle, i.e., different communication networks for different bit rate ranges would have to be provided given application of the STM principle, particularly given the currently desired broadband signal transmission. A uniform digital broadband communication network (broadband integrated services digital network, BISDN) cannot be realized with the assistance of the STM principle.
The ATM transmission or switching principle (asynchronous transfer mode) is significantly more flexible compared to the STM transmission principle. According to this ATM principle, calls that contain 63 octets or bytes as payload information as a standard are transmitted instead of the time slots of the STM principle. These ATM cells are transmitted with a constant transmission rate dependent on the band width of the transmission medium. Dummy cells are used when no messages are to be transmitted. A xe2x80x9cheaderxe2x80x9d, which contains the control or address information for the corresponding cell, is attached to the information field of every cell, which contains the actual payload information.
FIG. 3a shows an illustration for explaining the ATM principle. As shown in FIG. 3a, a plurality of cells Z are successively transmitted (in the direction indicated by the arrow) from a sender to a receiver. Each cell comprises a header with address or control information as well as an information field with the actual payload information. According to the defined standard, the information field comprises 48 octets, and the header comprises 5 octets, so that each cell is formed by 53 octets or bytes. Additional (header) octets can be attached to this cell format, which are capable of being employed for the routing of the cell upon transmission of the cell from a sending subscriber to a receiving subscriber.
In newer ATM broadband transmission systems or communication networks, the data streams between the individual transmission and reception assemblies are optically transmitted via light waveguides. These ATM broadband communication networks allow an extremely high data throughput that cannotxe2x80x94due to technological limitationsxe2x80x94be processed by the switching elements that are thereby employed and that are usually fashioned in CMOS technology. To address this problem, the data to be transmitted are therefore supplied in parallel to transmission modules via a plurality of data lines and transmitted by the transmission modules serially multiplex via the light waveguides to reception modules, which in turn divide the serial ATM data stream onto corresponding, parallel data channels at the output side for further processing.
This principle is shown in FIG. 3b. An optical ATM link serving as transmitter receives digital data of a plurality of data channels K0-Kn. Further, the sender S is supplied with a clock signal T. Dependent on the clock signal T, the sender S thus respectively reads n+1 bits in in parallel, and converts these bits into a serial, multiplexed ATM data stream D having a correspondingly higher data transmission rate, and this data stream D is optically transmitted to a receiver E. This receiver E parallelizes the received, serial data streams D, and in turn outputs it in parallel via date channel lines K0-Kn of the output side together with a clock signal T.
It is apparent on the basis of the above description that the demultiplexing of the serial data stream D in the receiver E represents a specific problem. For demultiplexing the data stream D, the receiver E must known which bit of the serial data stream D is to be allocated to which data channel K0-Kn of the output side. For this purpose, known solutions provide that additional synchronization information be attached to the actual serial data stream D at the transmission side, these additional synchronization information being interpreted in the receiver E and defining the allocation of the digital information transmitted in the serial data stream D to the individual data channels K0-Kn of the output side. Thus, for example, additional synchronization information can be attached with the assistance of an encoding implemented in the sender S, particularly a block encoding. As a result of the block encoding in the sender S, a redundancy is attached to the actual serial data stream D, as a result of which the serial data rate of the data stream D rises. On the other hand, a relatively high circuit outlay is required in the receiver E in order to be able to interpret the synchronization information attached to the serial data stream D. This all results in, for example, no inexpensive standard lasers can be utilized for the transmission of the data of the input-side data channels K0-Kn.
An example for the demultiplexing of a serial data stream is disclosed in U.S. Pat. No. 5,579,324, in which the arriving bit stream is synchronized by a control block, resulting in a significant outlay in the demultiplexing at the reception side.
Furthermore, Swiss Letters Patent 682 277 discloses methods for the synchronization of a serial ATM bit stream, which particularly addresses how the cell boundaries of a serial ATM bit stream can be identified. However, how a demultiplexing of a serially transmitted data stream is to be efficiently undertaken at the reception side is not addressed in this reference.
The present invention is therefore based on the object of creating a transmission method for an ATM transmission system as well as a corresponding ATM transmission system, in which a receiver-side demultiplexing of the serially transmitted data stream is possible with the relatively simple circuit-oriented outlay. In particular, a correct demultiplexing of the serial data stream should be possible without attaching additional synchronization information and, thus, without attaching reduncancy.
According to the present invention, this object is achieved by a method for the transmission of data in an ATM transmission system, comprising the steps of supplying digital data of a specific plurality of data channels parallel to an input side of a sender, converting the digital data into data units that respectively comprise an identical plurality of bits from each of the data channels, serially transmitting individual the data units in a form of cells that are respectively composed of a specific plurality of the data units, each cell having a specific, characteristic bit sequence, receiving, by a receiver the serially transmitted data units, monitoring, by the receiver, the received data units for an occurrence of the characteristic bit sequence and, after identifying the characteristic bit sequence, identifying a first data unit of a cell corresponding to the characteristic bit sequence, successively dividing, beginning with the first data unit of the cell corresponding to the characteristic bit sequence, individual bits of each the data unit of the corresponding cell onto a plurality of parallel data channels of an output side of the receiver corresponding in number to the plurality of data channels of the input side of the sender and the bits of each the data unit are output parallel via corresponding the data channels of the output side.
This object is also achieved by an ATM transmission system comprising a sender that converts digital data of a specific plurality of data channels supplied to it at an input side into data units such that each data unit comprises an identical plurality of bits from each the data channel, and serially transmits individual the data units via a transmission medium in a form of cells, each the cell comprising a specific plurality of data units, each the cell respectively comprising comprises a specific, characteristic bit sequence, a receiver that receives the serially transmitted data units from the sender and monitors the data units for an occurrence of the characteristic bit sequence, the receiver, after detecting the characteristic bit sequence in the serially transmitted data units, determines a first data unit of the cell corresponding to the characteristic bit sequence and, beginning with the first data unit, successively divides individual the bits of each the data unit of a corresponding cell onto a plurality of parallel data channels of an output side corresponding in number to the plurality of data channels of the input side and outputs the individual the bits of each the data unit in parallel.
According to the present invention and in agreement with the Related Art, the digital data of the parallel data channels present at the transmission side continue to be converted bit-by-bit into a serial ATM data stream, i.e., continue to be multiplexed, where the serial data of the ATM data stream are transmitted in the form of the initially described ATM cells. According to the present invention, however, a characteristic bit sequence with whose assistance the beginning of the corresponding ATM cell in the serial data stream can be acquired at the receiver side is transmitted within each cell. This characteristic bit sequence is preferably a matter of a synchronous octet that is already transmitted with every ATM cell, so that the beginning of the corresponding ATM cell can be recognized by monitoring the received data stream for the appearance of thin synchronous octet, and, thus, the information of the serial data stream can be correctly parallelized and divided onto corresponding data channels of the output side.
To this end, the digital data of the data channels supplied parallel at the input side are combined bit-by-bit into data units that form the ATM cells to be respectively transmitted. Each ATM cell transmitted with the assistance of the serial data stream thus contains a plurality of data units that respectively comprise an identical plurality of bits of each and every parallel data channel. It is fundamentally conceivable that two or more bits are transmitted with each data unit from each data channel. In practice, however, the parallel data channels adjacent at the input side are sampled bit-by-bit, so that each data unit of each data channel comprises only one bit. The corresponding bit of a data channel is always situated at the same location within each data unit, so that the individual bits can be easily divided onto the parallel, output-side data channels at the reception side after identification of the beginning of a data unit. The employment of respectively four data channels of the input side and output side is especially advantageous since the data of the data channels can be combined into half-bytes in four-bit fashion, by which each half-byte forms an above-described data unit of the ATM cell to be transmitted. Each octet of an ATM cell, accordingly, comprises two of these half-bytes. The data of each ATM cell are thus serially transmitted from the transmitter to the receiver in half-byte fashion.
The inventively proposed evaluation of the characteristic bit sequence of the cell, which is already transmitted with the cell and is usually formed by the first byte of each ATM cell, thus makes it possible that no additional signals or synchronization information for the channel allocation are required for the demultiplexing of the receiver side. An increase in the data rate of the optically transmitted, serial data stream together with the above-described associated disadvantages connected can thus be avoided. The invention thus enables a data transmission according to the ATM transmission principle with relatively little circuit outlay and allows the employment of smaller module sizes for the transmitter or receiver modules. Furthermore, the transmission is possible with a lower dissipated power, and the costs can be reduced as a result of the lower circuit outlay.
The invention is particularly directed to the transmission of data within an ATM switching system.